T-Mobile is GSM but Spring is CDMA.
Basically if you are the original owner of the Sprint account (phone number) you should contact Sprint and ask them to release your PIN number then contact your carrier that you are using on GSM (T-Mobile) and register it with your BIS account.

If you got the Blackberry from another Sprint user then contact him and ask him to call sprint and release the PIN number.

You need a Blackberry Data plan to use data services with your Blackberry... you say that you have one - so contact your carrier and register your PIN number with your BIS account and then update the service books on your unlocked Blackberry and have fun